1. Field of the Invention
The invention relates to a motor drive circuit that includes an inverter circuit, and an electric power steering apparatus.
2. Description of the Related Art
In an electric power steering apparatus that includes a motor drive circuit, in an abnormal situation where an inverter circuit cannot be used, an alternating-current motor is rotated in accordance with a steering operation performed on a steering wheel. In this case, the alternating-current motor serves as an electric power generator, and, for example, a battery (direct-current power source) of a vehicle is charged with the electric power generated by the alternating-current motor. In this case, the steering resistance includes the resistance for generating electric power by converting kinetic energy into electric energy (hereinafter, referred to as “power generation resistance”).
FIG. 5 shows a motor drive circuit with which the power generation resistance is excluded from the steering resistance. In the motor drive circuit shown in FIG. 5, a pair of interrupting MOSFETs connected in series is arranged in a feed line that connects the positive electrode of a direct-current power source to an inverter circuit. These interrupting MOSFETs are connected to the feed line in such a manner that parasitic diodes thereof are in the opposite orientations. Therefore, the situation where an electric current flows through both the parasitic diodes is avoided.
The interrupting MOSFETs are turned on and off by respective drive circuits. Each drive circuit is formed by connecting a voltage divider circuit and a drive MOSFET in series in this order in the direction from the source of the interrupting MOSFET toward the negative electrode of the direct-current power source, and connecting the output of the voltage divider circuit to the gate of the interrupting MOSFET.
In a normal situation, a control circuit continuously transmits gate signals to the drive MOSFETs to keep the interrupting MOSFETs on so that conduction between the direct-current power source and the inverter circuit is permitted. In contrast, in an abnormal situation where the inverter circuit cannot be used, transmission of gate signals to the drive MOSFETs is stopped so that the interrupting MOSFETs are turned off. Thus, conduction between the direct-current power source and the inverter circuit is interrupted so that the power generation resistance of the alternating-current motor is excluded from the steering resistance.
For example, Japanese Patent Application Publication No. 11-146558 describes a technology in which a pair of MOSFETs connected in series is arranged in a power line in such a manner that parasitic diodes thereof are oriented in the opposite directions and conduction through the power line is interrupted by turning off the MOSFETs.
However, in the existing motor drive circuit described above, if the positive electrode and the negative electrode of the direct-current power source are erroneously connected in reverse to the motor drive circuit, gate voltage is supplied to the interrupting MOSFETs through the drive circuits and the interrupting MOSFETs are turned on, whereby a closed circuit including the direct-current power source and the inverter circuit is formed. Then, a short-circuit current flows through the closed circuit as indicated by arrowed dot lines A, and the terminals of the direct-current power source are short-circuited.